CAD MODELLING AND RAPID PROTOTYPING AS TOOLS IN M...
1 di 4
file:///E:/webpages/HTML/432-Tornincasa.html
INTERNATIONAL CONFERENCE ON ENGINEERING DESIGN, ICED01 GLASGOW, AUGUST 21-23, 2001
S Tornincasa and E Chirone Keywords: CAD modelling, Rapid Prototyping, Medical applications
The manufacturing world is actually the main field for the application of CAD methods and Rapid Prototyping (RP) technology, but these are more and more widely used in medicine too. The development of appliances is chiefly related to producing either models for inspection use or prosthesis, directly or by a mould casting process. This paper describes the procedure followed to design and build a model of a cranial prosthesis, aimed to remedy a large defect due to a trauma. The reduced manufacturing times and good results obtained in terms of precision of the model are the basis to develop further procedures towards the construction of a real prosthesis for direct use.
The procedure to obtain suitable models is based on these steps: data acquisition, i.e. obtain exact definition of the body part to model transform the data in a CAD 3D model manipulate the CAD model to the most accurate representation transform the 3D model in a Stereo Lithography file (STL file) to interface with RP equipment obtain the solid real model by a suitable RP technique use the RP model for the planned purpose Computerised Tomography (CT) or Magnetic Resonance (MR) technologies make the first step easier by the scanning of anatomic parts. Computerised Tomography is based on a suitable positioning of the radiological set that is able to produce an image of the tissues focused only at a welldefined depth and so obtain welldefined sections of the internal organs and bone structures, allowing for a more accurate and complete analysis with respect to classical radiology. Thanks to the sections obtained by CT, usually with a thickness of 13 mm, it is possible to build 3D models of the examined organs. Some software houses have created procedures able to transfer information contained in the CT data directly to the RP machines, with the possibility of obtaining quickly and without difficulties solid replicas of complex parts. Such software is generally developed for the use of medical doctors, therefore they have simplified and not flexible interfaces and when the purpose is not the mere replication of an anatomic part, but its correction, deformation or simplification, it is evident that the use of such automatic software is not adequate. It is necessary then to use advanced CAD software, able to handle a complex geometry with modelling functions that enable the design of complex parts,
22/08/2014 12.12
CAD MODELLING AND RAPID PROTOTYPING AS TOOLS IN M...
2 di 4
file:///E:/webpages/HTML/432-Tornincasa.html
like the anatomical parts, and to convert it into an STL file. (Of course the use of such advanced software requires that a CAD technician should be added to the medical team). [1] The procedure for the creation of a solid model from a CT scanning so goes on with the data digitalisation and the conversion in a format compatible with the most used CAD software (usually the IGES format). The IGES files imported in the CAD environment contain the points forming the contour lines of the anatomical parts laying on different sections. (Fig. 1, left). The data of each section must be correctly dimensioned and the images laying on each plane should be perfectly piled up, an operation that, if not automatically made by the acquisition system, must be carried out using suitable reference points. Fig. 1 : The solid model originates from the scanned profiles The section contour lines are approximated by the use of spline curves, the more corresponding to the original profile the higher is the number of points. All the curves have a versus and an origin: in order to avoid the formation of structures not corresponding to the original, the versa of the curves must concord and the origins must be piled up. For this purpose, the CAD software must allow the operator to manipulate the versus and the origin of the curve. Once the spline curves necessary to define the model contours are obtained, two techniques can be used for the creation of the surfaces or, directly, of the solid: I) extrusion of each contour by a length equal to the scanning interval; II) generation of a surface interpolating two or more subsequent curves. The interpolation method generates aesthetically valid profiles; sometimes, however, the modellers cannot interpolate a high number of curves due to high computational load. Once the solid is generated, it is possible to obtain the corresponding STL file. The RP models then can be built, for different uses, as mentioned in the introduction. The production can be based either on Laminated Object Manufacturing, or on Stereo Lithography Apparatus (SLA), or even on Selective Laser Sintering (SLS) . The last method appears useful to obtain the best models for investment casting or to furnish exact parts for immediate use. The direct production of prosthesis by RP techniques, however, is limited by the availability of materials with sufficient mechanical strength and/or biological compatibility with the human tissues. For this reason, the RP models are so far being used mainly for the realisation of moulds, by which the real prosthesis are obtained.
This paper describes the procedure to design and build a metal model of a cranial prosthesis tailored on the case represented in fig. 2 (in a graphic reconstruction of the real skull), as consequence of a trauma caused by a car accident. Fig. 2 : The example model The CAD software chosen for 3D model generation was Solidworks , for its good ost/value ratio and portability, since it is designed for IBMcompatible personal computers and can run on Microsoft Windows 95/98/NT. The modelling operation has been carried out on a PC Pentium 350, with RAM of 256 MegaBytes. The lack in Solidworks of an importing filter for IGES format files was overcome by the design of conversion software, written in Visual Basic. A filter was added for the suppression of the curves formed by a limited number of points, not essential for the modelling and memory wasting. However, the conversion from IGES files and the processing of the spline curves required about
22/08/2014 12.12
CAD MODELLING AND RAPID PROTOTYPING AS TOOLS IN M...
3 di 4
file:///E:/webpages/HTML/432-Tornincasa.html
nine hours of work to generate 700 curves, laying on 80 plans orthogonal to the scansion axis, with 2 mm thick layers. Fig. 3 : The first steps in CAD reconstruction by SolidWorks The next step consisted in generating the curves needed to define the prosthesis profile. Such curves have been designed on each scansion plan, trying to approximate the natural curvature of the human cranium. Fig. 4 : The further steps in Solid Modelling In order to obtain a valid model of the original profile of the skull, a mirroring operation has been carried out over the curves of the intact part of the cranium, with respect to the symmetry plan of the human body. The next step was the interpolation of all the generated spline curves in order to obtain a solid model with the desired shape and subsequently the corresponding STL file. Under indication of the medical team, some chamfered holes have been added to the prosthesis design, in order to allow to the surgeon to perform the sutures and three brackets have been also added to allow to fix the prosthesis to the cranium by means of surgical screws. (Fig. 5) Fig. 5 : The solid model
A cast metal model of the cranial prosthesis has been manufactured in the laboratory of the company Technimold of Genoa. (Actually a model for a preliminary evaluation was made in the authors' Department by a ModelMaker system, [2], Fig. 7). As a first step, an ABS solid model of the prosthesis was manufactured, by means of a Stratasys FDM 8000 machine, and used as a model for a mould for casting in a metal alloy. For cost reasons, and since the results of the process could also be evaluated with Fig. 6 : The solid model in place: The assembly test has evidenced the good quality of the produced model: the curvature of the skull is effectively recovered and the contours of the bone defect to be sealed are respected with enough accuracy an alloy different from titanium (necessary for a real use), the metallic replica has been produced in pewter, for its low melting point and high ductility, that allows for modifications. The Spincasting process has been used to produce the metallic part. (Fig. 7) Fig. 7: The replicas (pewter at left, ModelMaker product at right) Fig. 8 : The pewter replica in place The pewter replica too is shown in fig. 7. The same replica put in place on a cranium model for a better simulation of the assembly is shown in fig. 8. Possible interference or fissures, due to scarce precision of the stereolithographic replica or of the CAD model, can be eliminated during the surgical operation by milling of the cranial bone to eliminate the interferences or by means of biocompatible mixtures to seal the fissures.
Of course the development of hardware and software information technologies will contribute to a reduction of the data acquisition and CAD modelling times, but at this moment the results obtained have shown some evident advantages of the application of RP techniques to the medical field. The main advantage stands in the high quality of the anatomical parts produced with RP techniques, characterised by good surface quality and geometrical accuracy, relatively to the needs of the medical applications. Furthermore, the manufacturing times are minimised on a few hours. Some disadvantages were however found, correlated to the recent
22/08/2014 12.12
CAD MODELLING AND RAPID PROTOTYPING AS TOOLS IN M...
4 di 4
file:///E:/webpages/HTML/432-Tornincasa.html
development of the process, that requires yet some adjustment, and other difficulties arise from the limited possibility of making to day the CAD modelling process of a personalised prosthesis manfree, without a continuous adjustment by specialised CAD operator in cooperation with medical staff. Another way to extend to medical applications the methods used in industrial design practice, is a more extensive investigation on Virtual Reality techniques. The VR approach allows evaluating not only the fitness of a prosthesis but also to simulating the operation scene. [4]. It appears useful also to have at the user's disposal a set of parts, based on parametric modelling, as reference in the case were the parts are little different, as in orthopaedic appliances Further development of this work are so expected, in cooperation with other research teams either in medical field (University of Turin) as in Engineering Departments (University of Udine).
[1] Bianchi S.D., Ramieri G., ``3D Tecniche di visualizzazione e replicazione solida -Applicazioni mediche''; Edizioni Minerva Medica, Torino, 1996 [2] Galter S., Settineri L., Tornincasa S., `` Building process of cranial prosthesis by means of RP techniques'', Proc. 4.th Convegno AITEM, Brescia, 1999, pp.5564 [3] Tornincasa S., Settineri L., Petkovic D., Ramieri G., ``Rapid Prototyping and CAD Modelling as the powerful tools in medical applications'', Proc. 5 th Int. Conference ``Trends in the Development of Machinery and Associated Technology TMT 2000'', Zenica, 2000, pp. 97102 [4] Filippi S., Bandera C., ``Il ruolo della RV nella pianificazione degli interventi di chirurgia ortopedica'', to be presented at the 13.th Int. Congress Ingenieria Grafica, Badajoz, 2001 Emilio Chirone Dip. Ingegneria Meccanica, Università di Brescia, Via Branze 38, 25123 Brescia, Italy Tel. ++39 030 3715423, Fax ++39 030 3702448, email
[email protected]
22/08/2014 12.12
